1. Field of the Invention
This invention relates to a prefastenable torque-shear bolt that can be mass-produced at low cost.
2. Description of the Prior Art
Prefastenable torque-shear bolts are used in the field of civil engineering and building where steel structural members to be connected are initially prefastened and afterwards are fully tightened with predetermined forces. The desired tightening force is obtained when a break-off groove cut in a bolt shears off when the designed torque is attained. Accordingly, each bolt is cut with two break-off grooves having fracture surfaces of different sizes, one spaced from the other along the axis of the bolt.
FIG. 1 shows an example of a conventional torque-shear bolt, which comprises a prefastening grip 2 and a full-fastening grip 3. The prefastening grip 2 is connected to the full-fastening grip 3 through a prefastening break-off groove 4, while the full-fastening grip 3 is connected to a threaded portion 6 through a full-fastening break-off groove 5. The effective area of the fracture surface of the prefastening break-off groove 4 is smaller than that of the full-fastening break-off groove 5. With this conventional torque-shear bolt 1, the prefastening grip 2 and the full-fastening grip 3 are of the same shape. Furthermore, the diameter D of a circle circumscribed around a polygon 8 on the prefastening grip 2 defined by rugged surfaces 7 with which a wrench engages is equal to that of the full-fastening grip 3, and the diameter d of a circle inscribed in a polygon 8 on the prefastening grip 2 is equal to that on the full-fastening grip 3 (as described, for example, in Japanese Provisional Utility Model Publication No. 52-128873 of 1977). When the prefastening break-off groove 4 is formed by rolling, burrs 10, whose total volume is equal to the volume of the steel 9 removed from the groove, are formed at the left end of the prefastening grip 2 and at the right end of the full-fastening grip 3 as shown in FIG. 3. Similar burrs are also produced when the full-fastening break-off groove 5 is formed. Consequently, the finished bolt 1 has the burrs 10 projecting on both sides of the two break-off grooves 4 and 5.
When joining together two steel plates 17 and 18 with a bolt 1 and a nut 11 using a tightening device 13, as shown in FIG. 4, the nut 11 is turned after fitting an inner socket 14 to the full-fastening grip 3 and an outer socket 15 to the nut 11. On this occasion, the inner socket 14 can be fitted over the prefastening grip 2. On completion of prefastening, however, the inner socket 14 cannot proceed to engage with the full-fastening grip 3 because of the burrs 10 standing in the way, as a consequence of which the bolt 1 fails to perform the desired function.
To remove such detrimental burrs 10 resulting from the roll-forming of the break-off grooves 4 and 5, a separate additional machining process is needed. This has not only hampered efficient mass-production but also has pushed up the production cost.